(1) Field of the Invention
The invention to a method of fabricating a transparent electrode, such as indium tin oxide for a thin film transistor.
(2) Description of the Prior Art
The indium tin oxide film, which is generally identified as ITO is composed of indium oxide and tin oxide. There are variations in the composition in regard to the completeness of the oxidation of the indium and tin plus the particular proportions of indium and tin oxide in the film. The principal methods of forming the ITO film is by sputtering either D.C. or R.F. The film may be deposited by simply sputtering a target of the desired composition or it may be deposited by a reactive sputtering technique wherein indium and tin from the target are reacted with oxygen in the ambient of the chamber. The Y. Endo et al U.S. Pat. No. 4,399,015 describes these types of deposition techniques.
The formation of thin film transistors require the use of various metals and insulators layers. ITO is a very desirable conductive layer in thin film transistors (TFT), because of its low resistivity and high transparency. ITO is preferably used for the pixel electrode of a LCD, because of its low resistivity and high transparency. Usually, the ITO is used together with a TFT. In this way, we can control the on and off of the LCD pixel by switching the TFT which is connected with the pixel electrode (ITO). Other materials that may also be used advantageously are, for example polysilicon, silicon oxide, aluminium with or without a silicon dopant, silicon nitride and the like.
A major drawback of ITO use is its processing relationship with these other materials. These problems stem in the most part from the difficulty of etching ITO and the difficulty for some materials to adhere to ITO. Some workers have even chosen to form an incomplete oxidized ITO which is then in turn etched and then annealed to fully oxidize the ITO as described by F. Boulitrop et al in U.S. Pat. No. 4,697,331. Additionally there can be processing of other materials which can damage the ITO itself. For example, since ITO is in a totally oxidized state, it is difficult to etch with the usual etchants. A very strong etchant is necessary, such as hydrochloric acid which has a detrimental effect upon other important layers in the thin film transistor structure. Also, there is very poor adhesion of aluminium to ITO (although the reverse, ITO to aluminium adhesion is satisfactory). The deposition of silicon nitride by plasma enhanced reactor can cause damage to the ITO.
It is therefore, an important object of this invention to provide a method for fabricating a thin film transistor LCD which uses ITO in the proper sequence to overcome the various problems associated with this material.